Aircraft such as passenger aircraft comprise a number of powered systems, such as control surface actuators, passenger air systems and avionics. In traditional aircraft, some of these systems are electrically powered, while others are pneumatically or hydraulically powered. Aircraft engines, such as gas turbine engines, also comprise a number of powered accessory systems, such as fuel and oil pumps, which are powered by the engine itself. In recent years, there has been a move to provide electrically powered systems for aircraft and aircraft engine accessory systems to replace some or all of the pneumatic or hydraulic systems in order to save weight or reduce fuel burn, or a combination of the two. Aircraft having more electrical systems compared to traditional aircraft are known as “More Electric Aircraft” (MEA), whereas aircraft engines having more electrically powered accessory systems are known in the art as “More Electric Engines” (MEE).
In the case of an MEE, where the engine accessory is required for continued operation of the engine, such as where the engine accessory comprises the fuel or oil pump, a fault within the electrical system may result in an undemanded drop in power and consequently a loss of thrust control, or possibly an engine shutdown. It is usually a requirement of engine and aircraft certification that a single electrical failure in a single component does not result in loss of thrust control or an engine shutdown. Consequently, where essential engine accessory systems are electrically powered, such accessory systems may require redundancy, i.e. multiple independent systems, in order to prevent a fault with a single system from resulting in loss of thrust control. Such redundancy however increases weight and costs to the extent that the advantages of an MEE may not be realised.
In a separate field of endaevaour, fault tolerant electrical machines are known. Fault tolerant electrical machines are electrical machines which are configured to continue working even if a failure occurs. One type of fault tolerant electrical machine comprises a multi-phase alternating current (AC) electrical machine, such as a generator or motor. “Fault Tolerant Electrical Machines—State of the Art and Future Directions” by Mircea Ruba and Lorand Szabo, published by the Technical University of Cluj, describes an electrical machine control system for controlling a fault tolerant switched reluctance electrical machine (SRM). When a fault condition such as an open or short circuit is sensed, the controller increases the current in the remaining phases to compensate. However, in order for the electrical machine to able to continue operating in the fault condition, it is necessary to rate each phase to cope with the power and cooling requirements for the fault condition. Consequently, conventional fault tolerant electrical machines must be relatively large, and therefore heavy and expensive.
The present invention describes an aircraft electrical system and a method of controlling an aircraft electrical system which seeks to overcome some or all of the above problems.